The present invention relates to an AD conversion method and a system for converting analog signals into digital signals. The AD conversion method or the system according to the present invention can be effectively used, for example, in a digital waveform storage device or a digital storage oscilloscope.
In the conventional digital storage oscilloscopes and digital waveform storage devices as disclosed, for example, in Japanese Patent Application Laid-Open No. 59-37730, the input analog signals are converted into digital data through an AD converter (hereinafter referred to as ADC), the digital data is stored in a memory at a high speed, the stored data is retrieved from the memory into a microprocessor or a central processing unit where it is subjected to signal processing and display on the screen of a cathode-ray tube.
In this case, in general, a single ultra-high speed ADC is used.
According to the above prior art, the conversion operation is usually delayed when the input analog signal frequency f.sub.in and the sampling frequency f.sub.s exceed given values, and erroneous digital codes are produced. This phenomenon is called conversion error or a missing code.
The conversion error develops more frequently with the increase in the resolution of the ADC, at lower frequencies of input analog signals or of sampling signals.
In a 4-bit ADC, for example, a sampling frequency f.sub.s of 500 megasamples/sec. (Msps) is permitted. In an 8-bit ADC, however, a sampling frequency f.sub.s of only about 200 Msps is permitted.
As mentioned above, the conversion error imposes a limitation on the input signal frequency f.sub.in and on the sampling frequency f.sub.s, making it difficult to carry out the AD conversion at high speeds.